Any tappet smaller than .990", including the "normal" Chrysler .904", requires the bore to be sleeved. The only big cam that will run on the .990" tappet is something designed for the Chrysler .970" mushroom.
AFAIK, the reason for a precision tool to spot-face the bottom of the tappet bore is tight conditions in a V8 valley area - the GMC should have room to get a bigger tool in there. However, I'm a little nervous about how much metal there is above the cam. To fully recess the VW std. 31mm tappet, the bore radius is widened by .115". On the outboard side, the skirt wall looks like only 3/16" thick.
